The nitrogenase system of Clostridium pasteurianum is being investigated to determine how it catalyzes the ATP-dependent reduction of dinitrogen. The enzyme has been separated into its components, molybdoferredoxin and azoferredoxin and these are being studied both separately and in a recombined system. Both components have characteristic electron paramagnetic resonance signals that change in a defined manner on addition of substrates and these changes are being used to monitor the reactions of nitrogenase. In addition experiments to determine the role of the metals of the nitrogenase components, Fe and Mo, are in progress. The effect of magnesium ATP on azoferredoxin is being further studied since present data indicate it is undergoing a conformational change. Finally we are studying how the nitrogenase reactions are controlled by magnesium ADP and by carbamyl phosphate. Both inhibit activity but only the latter represses synthesis.